Field of the Invention
This invention relates to a method of migrating a melt of a metal through a solid body of semiconductor material by thermal gradient zone melting (TGZM) and, in particular, to the uniform initiation of migration by enhancing the penetration of melts into the material at the surface when fine liquid lines or small diameter liquid droplets are being migrated.
Description of the Prior Art
W. G. Pfann in U.S. Pat. Nos. 2,739,088 and 2,813,048 describes methods for practicing the movement of melts of metal through particular regions of a solid body of semiconductor material by thermal gradient zone melting. However, molten line and droplet in stability resulted in the breakup of the migrating lines and droplets and consequently acceptable semiconductor devices were not always obtainable.
Recently, we discovered that preferred planar orientation of the surfaces of the body of semiconductor material, migration axis and line orientation axis relationship were also a necessity to migrate liquid metal wires and/or droplets through the solid body. (See U.S. Pat. Nos. 3,899,362 and 3,904,442, for example). These improvements in TGZM resulted in commercialization of the process. However, as the width of the lines being migrated became smaller, the penetration of fine liquid lines of less than 2 mils in width, and preferably 1 mil in width, and small liquid droplets, less than 6 mils in diameter, from the surface of a wafer or body of semiconductor material has been difficult to achieve repeatedly, on a commercial basis, by a thermal gradient alone. At wire sizes less than 30 microns it becomes difficult to migrate in a straight path and so produce straight uniformly doped trails particularly at temperatures below 1000.degree. C. One reason noted for this difficulty is that the migration direction deviates from the applied thermal gradient in the surface region of the body where migration is initiated. Although the thermal gradient is strong enough to cause migration of the small liquid zones once they are formed in the bulk of semiconductor material, the thermal gradient force is not powerful enough to overcome the surface tension forces holding fine liquid zones, or wires, on the surface of a body, or wafer. Further improvements to the TGZM processing techniques included alloying the deposited metal to the surface (U.S. Pat. No. 3,897,277) and sintering of the same (U.S. Pat. No. 4,006,040). The problem still persists as one attempts to migrate fine lines and droplets on a commercial basis. As a result, TGZM to date has been limited to line and droplet dimensions typical of solid-state power device isolation grids and has not had any commercial impact on integrated-circuit type devices which require a much finer size of doped region.
Presently the preferred source of radiant energy for practicing thermal gradient zone melting is infrared lamps. The major surface on which the metal layer to be migrated is deposited is exposed to the heat sink. The other major surface is exposed to the infrared lamps. The radiant energy is directed onto that major surface, travels through the wafer, or body, and is emitted at the other major surface where the metal is deposited. The body is almost transparent to much of the radiant energy of the lamps. Consequently, the region of the body, including the major surface with the metal deposit, has substantially no thermal gradient therein in the vertical direction necessary for migration. Therefore, although the radiant energy heats the body sufficiently to melt the metal, little if any penetration of the melt into the surface may occur and migration is sluggish, if at all. Therefore any lateral gradients present in the body will affect the initiation and/or vertical melt migration. In the region of almost no vertical thermal gradient, the effect becomes more pronounced as the wire lines become smaller in width, and initiation of vertical melt migration may never occur. In migrating grids for some discrete devices, the wire lines migrated are of the order of 250 microns. Consequently, the effect of little or no vertical thermal gradient in the affected region is little noticed.
Fine liquid wires of less than 10 microns and droplets of less than 100 microns in diameter experience difficulty in penetration and initial migration. If a lateral thermal gradient is present, the fine line or droplet may slide along the surface until vertical melt migration occurs. Should vertical melt initiation occur, melt migration may still proceed off-axis to the vertical gradient under the influence of the lateral gradient. After awhile, vertical gradient becomes the dominant influence and migration proceeds substantially vertical to the two opposed major surfaces.
Therefore, it is an object of this invention to provide a new and improved method to migrate fine molten lines and droplets of metal through a solid body of semiconductor material by thermal gradient zone melting (TGZM) processing which overcomes the deficiencies of the prior art.
Another object of this invention is to provide a new and improved method for initiating the penetration of fine molten metal wires and droplets into the surface region of a solid body for migration therethrough by TGZM by providing a heavily doped region in the body which includes the surface from which initiation of the melt is practiced.
A further object of this invention is to provide a new and improved method for maintaining the thermal gradient through the entire body to keep the melt migration velocity essentially constant.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.